Numerous complex and sensitive chemical and mechanical processes requiring complicated, delicate and specialized equipment are necessary to fabricate a modern IC. A modern IC fabrication facility therefore must be carefully maintained and monitored to ensure that the ICs it produces operate as intended. Monitoring and maintaining an IC fabrication facility under normal circumstances are sufficiently challenging. However, they becomes especially challenging when changes are introduced into the facility.
Changes come in many different types and degrees. For example, a change may result when a new supplier is chosen to supply a gas used to fabricate ICs, when a process temperature, pressure or time is modified, when equipment is repaired, modified, replaced or added or when process steps are added or omitted. A customer may explicitly request that a change be made.
A responsible manufacturer evaluates changes to determine their effect on ICs being fabricated before incorporating those changes into regular production. The issue is one of “equivalency” or “sameness;” is an IC fabricated after a change the “same” as an IC fabricated before the change? If so, the change is deemed acceptable for regular production.
Equivalency is analyzed with respect to one or more measurable and quantifiable characteristics, e.g., voltage, current, speed or dimension. The one or more characteristics are measured with respect to a set of ICs fabricated by the process before the change (often called the “control” or “base” set) and a set of ICs fabricated by the process after the change (often called the “experimental” or “test” set). For purposes of the present discussion, the fabrication process without the change will be called the “normative” fabrication process, and the fabrication process that includes the change will be called the “candidate” fabrication process.
Base and test sets may be designated in different ways. For example, a single lot of ICs may be divided (typically evenly) into base and test sets; this is called a “split lot.” Alternatively, a single lot may be designated as the test set. Although split lots are generally preferred, the latter may be preferred when the change in question is customer-driven and the lot is therefore customer-specific (a so-called “limited release” lot). One or more split or limited release lots may be used to analyze equivalency with respect to a given candidate process.
Irrespective of whether split lots or limited release lots are used, one conventional process of analyzing equivalency is as follows. First, the lots are processed and one or more of their characteristics measured. Then, the characteristics of the test set (which form a scattered distribution) are compared to the characteristics of the base set (which also form a scattered distribution). Extreme values, or “outliers,” in each set are often disregarded. Equivalency is defined as the extent to which those sets of characteristics overlap. An overlap of 98% may, depending upon application-specific circumstances, be regarded as sufficient for equivalency to exist.
For example, if a lot of 100 wafers having 10 ICs per wafer is split into a 50-wafer base set and a 50-wafer test set, the characteristic(s) of 500 test ICs (50 wafers multiplied by 10 ICs per wafer) are compared to the characteristic(s) of 500 base ICs. If the two sets of characteristic(s) overlap (disregarding outliers) by at least a threshold extent (e.g., 99%), equivalency is regarded to exist.
U.S. Pat. No. 6,789,031, which issued to Wang on Sep. 7, 2004, entitled “Method for Determining the Equivalency Index of Products, Processes, and Services,” commonly assigned herewith and incorporated herein by reference, describes a process of analyzing equivalency.
The accuracy with which equivalency is determined can have a profound impact on the cost of ICs and the reputation of their manufacturer. Two types of errors are possible. False positives (sometimes colloquially called “false reds”) occur when a particular IC fails the equivalency test, but actually works properly. False positives are incorrectly regarded as waste and discarded; the cost of their fabrication must be recovered by other means. False negatives (sometimes colloquially called “false greens”) occur when a particular IC passes the equivalency test, but actually is defective. False negatives are shipped to customers as though they were good. The customers are then left to discover that one or more of the ICs for which they have paid are defective, which is a source of potential embarrassment to the manufacturer.
While reasonably adept at determining equivalency, the above-described conventional process sometimes still produces false positives and false negatives. Accordingly, what is needed in the art is a better way to determine equivalency.